There has been conventionally known an ultrasonic fatigue test that evaluates a fatigue life of a metallic material or the like by using an ultrasonic wave to oscillate a test piece. In such an ultrasonic fatigue test, stress is repetitively loaded on a test piece by resonating the test piece with, for example, sine wave oscillation at 20 kHz.
On the other hand, in the ultrasonic fatigue test, the high-frequency repetitive load is applied to the test piece, and therefore a rise in temperature of the test piece caused by the internal heat generation of a material becomes problematic. For this reason, there has been proposed an ultrasonic fatigue testing machine that measures temperature near a test piece during a test, and when the temperature reaches a predetermined temperature or more, sprays liquid nitrogen to cool the test piece (see Patent Literature 1).
Also, there has been proposed an ultrasonic fatigue testing machine that in order to suppress the internal heat generation of a test piece, periodically repeats excitation, which applies ultrasonic oscillation to the test piece, and a pause, i.e., performs so-called intermittent operation (see Patent Literature 2).